1. Field of the Invention
This invention relates generally to nucleic acid sequences encoding proteins that are associated with abiotic stress responses and abiotic stress tolerance in plants. In particular, this invention relates to nucleic acid sequences encoding proteins that confer drought, cold, and/or salt tolerance to plants.
2. Background Art
Abiotic environmental stresses, such as drought stress, salinity stress, heat stress, and cold stress, are major limiting factors of plant growth and productivity. Crop losses and crop yield losses of major crops such as rice, maize (corn) and wheat caused by these stresses represent a significant economic and political factor and contribute to food shortages in many underdeveloped countries.
Plants are typically exposed during their life cycle to conditions of reduced environmental water content. Most plants have evolved strategies to protect themselves against these conditions of desiccation. However, if the severity and duration of the drought conditions are too great, the effects on plant development, growth and yield of most crop plants are profound. Furthermore, most of the crop plants are very susceptible to higher salt concentrations in the soil. Continuous exposure to drought and high salt causes major alterations in the plant metabolism. These great changes in metabolism ultimately lead to cell death and consequently yield losses.
Developing stress-tolerant plants is a strategy that has the potential to solve or mediate at least some of these problems. However, traditional plant breeding strategies to develop new lines of plants that exhibit resistance (tolerance) to these types of stresses are relatively slow and require specific resistant lines for crossing with the desired line. Limited germplasm resources for stress tolerance and incompatibility in crosses between distantly related plant species represent significant problems encountered in conventional breeding. Additionally, the cellular processes leading to drought, cold and salt tolerance in model, drought- and/or salt-tolerant plants are complex in nature and involve multiple mechanisms of cellular adaptation and numerous metabolic pathways. This multi-component nature of stress tolerance has not only made breeding for tolerance largely unsuccessful, but has also limited the ability to genetically engineer stress tolerance plants using biotechnological methods.
Drought, cold as well as salt stresses have a common theme important for plant growth and that is water availability. Plants are exposed during their entire life cycle to conditions of reduced environmental water content. Most plants have evolved strategies to protect themselves against these conditions of desiccation. However, if the severity and duration of the drought conditions are too great, the effects on plant development, growth and yield of most crop plants are profound. Since high salt content in some soils result in less available water for cell intake, its effect is similar to those observed under drought conditions. Additionally, under freezing temperatures, plant cells loose water as a result of ice formation that starts in the apoplast and withdraws water from the symplast. Commonly, a plant's molecular response mechanisms to each of these stress conditions are common and protein kinases play an essential role in these molecular mechanisms.
Protein kinases represent a super family and the members of this family catalyze the reversible transfer of a phosphate group of ATP to serine, threonine and tyrosine amino acid side chains on target proteins. Protein kinases are primary elements in signaling processes in plants and have been reported to play crucial roles in perception and transduction of signals that allow a cell (and the plant) to respond to environmental stimuli. In particular, receptor protein kinases (RPKs) represent one group of protein kinases that activate a complex array of intracellular signaling pathways in response to the extracellular environment (Van der Gear et al., 1994 Annu. Rev. Cell Biol. 10:251-337). RPKs are single-pass transmembrane proteins that contain an amino-terminal signal sequence, extracellular domains unique to each receptor, and a cytoplasmic kinase domain. Ligand binding induces homo- or hetero-dimerization of RPKs, and the resultant close proximity of the cytoplasmic domains results in kinase activation by transphosphorylation. Although plants have many proteins similar to RPKs, no ligand has been identified for these receptor-like kinases (RLKs). The majority of plant RLKs that have been identified belong to the family of Serine/Threonine (Ser/Thr) kinases, and most have extracellular Leucine-rich repeats (Becraft, P W. 1998 Trends Plant Sci. 3:384-388).
Another type of protein kinase is the Ca+-dependent protein kinase (CDPK). This type of kinase has a calmodulin-like domain at the COOH terminus which allows response to Ca+ signals directly without calmodulin being present. Currently, CDPKs are the most prevalent Ser/Thr protein kinases found in higher plants. Although their physiological roles remain unclear, they are induced by cold, drought and abscisic acid (ABA) (Knight et al., 1991 Nature 352:524; Schroeder, J I and Thuleau, P., 1991 Plant Cell 3:555; Bush, D. S., 1995 Annu. Rev. Plant Phys. Plant Mol. Biol. 46:95; Urao, T. et al., 1994 Mol. Gen. Genet. 244:331).
Another type of signaling mechanism involves members of the conserved SNF1 Serine/Threonine protein kinase family. These kinases play essential roles in eukaryotic glucose and stress signaling. Plant SNF1-like kinases participate in the control of key metabolic enzymes, including HMGR, nitrate reductase, sucrose synthase, and sucrose phosphate synthase (SPS). Genetic and biochemical data indicate that sugar-dependent regulation of SNF1 kinases involves several other sensory and signaling components in yeast, plants and animals.
Additionally, members of the Mitogen-Activated Protein Kinase (MAPK) family have been implicated in the actions of numerous environmental stresses in animals, yeasts and plants. It has been demonstrated that both MAPK-like kinase activity and mRNA levels of the components of MAPK cascades increase in response to environmental stress and plant hormone signal transduction. MAP kinases are components of sequential kinase cascades, which are activated by phosphorylation of threonine and tyrosine residues by intermediate upstream MAP kinase kinases (MAPKKs). The MAPKKs are themselves activated by phosphorylation of serine and threonine residues by upstream kinases (MAPKKKs). A number of MAP Kinase genes have been reported in higher plants.